Pointer to pointer in parameter in C

Question

I need to make a array of strings (dynamic). I send a pointer to pointer to function, But in the function I got error.

This is the function:

void CheckFilesInFolder(char* pathOfFolder, char*** namesFiles);

I call to function like that:

char** namesFiles = NULL;
CheckFilesInFolder("Debug", &namesFiles);

And in the function i do that malloc, and when I = 1 its break.

*namesFiles = (char**)malloc(*namesFiles, 10 * sizeof(char*) );
int i = 0;
for (i = 0; i < 10; i++)
{
    *(namesFiles[i]) = (char*)malloc(MAX_FILE_NAME);
}

[Please see this discussion on why not to cast the return value of `malloc()` and family in `C`.](http://stackoverflow.com/q/605845/2173917). — Sourav Ghosh, May 14 '16 at 11:46
A *void* function with a `char ***` argument to return some dynamically allocated thing to the caller. USE THE RETURN VALUE, LUKE! — wildplasser, May 14 '16 at 11:50
You have a ton of other errors, but in principle [this approach works](https://ideone.com/UUFmkA). — Kerrek SB, May 14 '16 at 11:53

Luis Paulo · Accepted Answer · 2016-05-14T17:08:34.870

malloc only takes 1 size_t argument. Change this:

*namesFiles = (char**)malloc(*namesFiles, 10 * sizeof(char*) );

to this:

*namesFiles = (char**)malloc(10 * sizeof(char*) );

Let me know of any other errors that might occur.

Also, if you need to pass the pointer to the caller, why don't you return it instead of getting it by passing the pointer's adress as an argument?

Like so: char** CheckFilesInFolder(char* pathOfFolder);

and in the function do something like:

char** namesFiles = (char**)malloc(10 * sizeof(char*) );
//(...)
return namesFiles;

To call, just do this: char** namesFiles = CheckFilesInFolder("Debug");

I believe this is the best way.

EDIT 1.0 To reply to the comment from "alk", here is the way of solving this problem with a pointer to an array of char* through the arguments instead of my (simpler) suggestion to return a char**:

Call the function like this:

char** namesFiles;
CheckFilesInFolder("Debug", &namesFiles);

Inside the function:

*namesFiles = (char**)malloc(10 * sizeof(char*));
int i;
for (i = 0; i < 10; i++)
    (*namesFiles)[i] = (char*)malloc(MAX_FILE_NAME);

Function declaration:

void CheckFilesInFolder(char* pathOfFolder, char*** namesFiles);

Use this is you really need to do this using arguments for example if you want to call this function as the routine of a pthread.

You are right. Forgot he was using char***. Edited. It's an unnecessary complication, though. He could just return the char**. Thank you for the warning — Luis Paulo, May 14 '16 at 13:02
I feel this answer is not pointing to the relevant issues with the OP's code. — alk, May 14 '16 at 16:51
@alk Ok. Fair enough. It's answering the question exactly now. — Luis Paulo, May 14 '16 at 17:11

Pointer to pointer in parameter in C

1 Answers1